The operation of devices for locking/unlocking doors/windows of a vehicle today often appeal to devices known as hands-free devices, based on information exchange by radio channels between onboard equipment in the vehicle and a piece of electronic equipment, often called an identifier, carried by the user.
One of the particular modes of operation of such devices is mode D, called the approach-detection mode. This mode corresponds to a situation in which access to the vehicle is locked in the absence of an identifier close to the vehicle and a mode in which the onboard system seeks to detect whether an identifier, carried by a user approaching the vehicle, comes within a proximity perimeter inside which the presence of the identifier may be detected. It is therefore a mode in which the vehicle seeks to detect the approach of an identifier.
In order to detect whether an identifier comes within the proximity perimeter the onboard equipment frequently transmits radio signals, generally in a low frequency range, called LF, in the region of 125 kHz, which will be received by an identifier, if the identifier is within the proximity perimeter.
When the identifier enters within said proximity perimeter, it receives the LF radio signals transmitted by the onboard equipment and it in turn transmits a radio signal, generally in a radiofrequency range, called RF, in the region of 433 MHz, to inform the onboard system of its presence within the proximity perimeter. Obviously, for reasons of security, the exchanged signals are encoded to allow an exclusive exchange between an onboard system and an authorized associated identifier.
When the RF signal of an identifier is received by the onboard system, the device leaves the approach-detection mode D.
To carry out this approach-detection function, as illustrated in FIG. 1, the onboard equipment employs signal transmission means comprising external antennas 14a, 14b distributed over the vehicle to cover the proximity perimeter within which the LF signals have to be received by an identifier, amplification means 12a, 12b connected to the antennas and control means 10.
In approach-detection mode D, such a device transmits periodic signals in the expectation of a response from an assumed identifier, which transmission leads to electrical consumption.
The known devices absorb a power of around 2 W, partly at least due to the power radiated and due to the polarization currents linked with the technology of the amplifiers used.
When the device remains in approach-detection mode D for a long period, the battery essentially drawn on by the device is progressively discharged. It is frequently observed that a vehicle equipped with such a device cannot start on the battery if it has remained in approach-detection mode longer than a few days.